For example, cylinder blocks for use in automotive engines are made of an aluminum alloy for producing lighter engines. The cylinder blocks include cast-iron cylinder sleeves or liners (inserts) to provide wear-resistant inner surfaces against which pistons slide back and forth. Brake drums for automobiles also use cast-iron shoes (inserts).
When a metal, e.g., an aluminum alloy, is to be cast around a cast-iron insert, it is necessary that the cast-iron insert and the aluminum alloy be held in intimate contact with each other and that the aluminum alloy fill surface irregularities of the cast-iron insert. To meet such requirements, Japanese laid-open patent publication No. 2001-170755 discloses a cast-iron insert having surface irregularities whose maximum height ranges from 65 μm to 260 μm and whose average interval ranges from 0.6 mm to 1.5 mm.
According to the above publication, an aluminum alloy is cast around the outer peripheral surface of the cast-iron insert by a die-casting process to obtain a product where the aluminum alloy well fills the surface irregularities of the outer peripheral surface of the cast-iron insert and the cast-iron insert is held in highly intimate contact with the aluminum alloy.
To form the desired outer surface of the cast-iron insert, there is employed a facing material in the form of a suspension which contains a mixture of 20 weight % to 45 weight % of silica sand having an average particle diameter in the range from 0.05 mm to 0.5 mm, 10 weight % to 30 weight % of silica flour having an average particle diameter of 0.1 mm or less, 2 weight % to 10 weight % of a binder, and 30 weight % to 60 weight % of water.
After the inner surface of a heated mold is coated with the above facing material, the facing material is dried. When the facing material is dried, the facing material produces a vapor through holes therein, forming countless minute recesses in the inner surface of the mold. When molten cast iron is then poured into the mold, the produced cast-iron insert has an outer surface having spines corresponding to the recesses in the inner surface of the mold.
As shown in FIG. 9 of the accompanying drawings, a cast-iron insert 1 has an outer surface 3 having needle-like spines 2. When an aluminum alloy 4 is cast around the outer surface 3 of the cast-iron insert 1, a cast product 5 is produced. Since the outer surface 3 of the cast-iron insert 1 has a plurality of spines 2, the cast aluminum alloy 4 is prevented from being relatively displaced with respect to the cast-iron insert 1 in the directions indicated by the arrow A, and is subject to reduced residual stresses.
However, the cast-iron insert 1 peels off the aluminum alloy 4 in the directions indicated by the arrow B parallel to the spines 2. When the cast-iron insert 1 peels off the aluminum alloy 4, the cast-iron insert 1 is brought out of close contact with the aluminum alloy 4, and the area of contact between the cast-iron insert 1 and the aluminum alloy 4 is reduced, thus lowering the thermal conductivity of the cast product 5.
After the cast-iron insert 1 is manufactured by casting, the inner surface (sliding surface) of the cast-iron insert 1 needs to be machined. When the inner surface of the cast-iron insert 1 is machined, the outer surface 3 of the cast-iron insert 1 is clamped by a clamp mechanism.
Because the spines 2 project from the outer surface 3 of the cast-iron insert 1, the clamp mechanism has its clamping surface held in point-to-point contact with the tip ends of the spines 2. As a result, the area of contact between the clamping surface and the cast-iron insert 1 is relatively small. On account of the relatively small area of contact between the clamping surface and the cast-iron insert 1, the cast-iron insert 1 is not positioned accurately while the inner surface of the cast-iron insert 1 is being machined. Consequently, the inner surface of the cast-iron insert 1 cannot be machined accurately.